As a result of improvements in materials and technology, and the current trend towards miniaturization and portability, electronic and mechanical devices for personal and industrial use are becoming sensitive to an increasing number of materials that can adversely affect their performance and are increasingly being installed, or carded into, and used in environments which contain such materials.
When possible, such devices are contained in sealed enclosures and are not exposed to the ambient environment. However, in most cases, it is desirable for the enclosures and housings containing sensitive mechanical or electronic equipment to have openings that permit easy passage of gases, such as air, through them. This may be desirable for transmission of sound, for example, in the case of cordless telephones, transceiver radios, pagers, loudspeakers, and the like; or for the purpose of accomodating or controlling changes in temperature and pressure in devices such as modern cameras with auto-focussing or zoom lenses, computers, analytical instruments, automobile electronic controls, pressure sensors and pressure switches, and the like.
In these cases it is necessary to protect the sensitive components of the devices from contact with harmful elements present in the ambient environment. This is usually done by installation of a filter in the vents, or openings, in the enclosure to serve as a barrier to harmful particulates, liquids, or aerosols present in most environments. Porous membranes of hydrophobic materials such as polytetrafluoroethylene or other fluoropolymers, or polyolefin polymers, have been used as vent filter materials, and are well known in the art. Also, other synthetic polymers in the form of woven or non-woven fabrics, mesh, netting, or felts treated with water-repellent coatings have been used as water-repellent gas-permeable filter materials. Filters containing these materials can provide good barrier properties against passage of particles, water, or water mist. However, such materials generally lack oil-repellency and, when contacted by oils, may become plugged or blinded so that gas-permeability is lost; or if contacted with many water-soluble oils, detergents, surfactants, and the like, may lose their water-repellency.
Because of the ubiquitous presence of oils and their aerosols in many environments, for example, lubricating oil and grease, power steering fluid, brake fluid, etc., in automotive environments; cooking oils, food fats and oils, kitchen detergents, human body oils, etc., in home, business, and restaurant environments; and the many oils, solvents, surfactants, and other chemicals, etc., in industrial environments; it has become desirable to have filters that also have oil-repellent properties. U.S. Pat. No. 5,116,650 (to Bowser) discloses porous materials which have oleophobic properties developed in them by coating the pore interiors with a coating of an amorphous copolymer of tetrafluoroethylene and perfluoro-2,2-dimethyl-1,3-dioxole.